There are several known methods and apparatus for pumping high consistency pulp. Previously only displacement pumps, such as screw pumps or like, were used to pump high consistency pulp. Nowadays there is a tendency to replace the displacement pumps because of their inherent deficiencies and drawbacks. One of the first problems encountered when trying to pump pulp with the consistency of more than 8% is that the pulp does not independently flow to the impeller of the pump in the suction opening. A solution to this problem is a so-called fluidizing centrifugal pump, manufactured and sold by A. AHLSTROM CORPORATION of Karhula, Finland and by AHLSTROM PUMPS, INC. of Peace Dale, R.I. There fluidizing pumps are designed to treat medium and high consistency pulps by the action of the fluidizing rotor extending into the suction opening of the pump or in some cases through it as far as into the mass tower. By using this kind of fluidizing rotor it has been possible to pump pulp having a consistency of about 15%, which does not, however, satisfy all requirements for pulp pumping in the pulp and paper industry, as the consistency demands have risen up to about 25%.
Another difficulty in connection with the pumping of medium and high consistency pulps is that pumping of fluids containing gases, with higher gas contents, is unsuccessful without a gas discharge system because the gases collect in front of the center of the pump impeller forming a bubble which will grow thus tending to clog the entire inlet opening of the pump. This results in a considerable decrease of the yield, vibration of the equipment, and in the worst case ceasing of the pumping action altogether. This problem has been experienced in a very intense form with, for example, centrifugal pumps.
These problems have been attempted to be solved in many different ways by discharging gas from the bubble. In the equipment presently known and used, degasification is effected by either drawing gas through a pipe being disposed in the middle of the inlet channel of the pump and extending to the hub of the impeller, by drawing gas through a hollow shaft of the impeller, or by providing the impeller with one or more perforations through which the gas is drawn to the back side of the impeller and further away by some kind of a vacuum device arranged usually outside the pump.
Several different arrangements are known by means of which it has been attempted to eliminate or minimize the disadvantages or risks caused by contaminants. The simplest arrangement is a gas discharge duct which is so wide that clogging thereof is out of the question. Other methods used are, for example, arrangements with various types of vanes or vaned rotors on the back side of the impeller. A commonly used method has been to provide the immediate back surface of the impeller with radial vanes for pumping the fluid together with its contaminants. Thereby the fluid is carried with the gas through the gas discharge openings of the impeller, to the outer periphery of the impeller and through its clearance back to the liquid flow. In some cases, a similar arrangement has further been provided on the back side of the impeller with a vaned rotor mounted on the shaft of the impeller. The vaned rotor rotates in a separate chamber, being adapted to separate the liquid, which has been carried with the gas, to the outer periphery of the chamber, whereby the gas is drawn to the inner periphery thereof. The fluid accumulated at the outer periphery of the chamber is led, together with the contaminants, through a separate duct to either the inlet side or the outlet side of the pump. The gas is removed from the inner periphery by means of suitable vacuum device.
As can be seen all centrifugal pumps for pumping medium or high consistency pulps require some gas separation or discharge device which is most often arranged outside the pump as an entirely separate unit. All means described above operate satisfactorily if the amount of contaminants carried with the liquid is somewhat limited. It is also possible to adjust the pumps to operate relatively reliably with liquids containing large amounts of solids, e.g. with fiber suspensions in the pulp industry. It is known that the gas contained in the fiber suspension is a drawback in the stock preparation process. Accordingly this drawback should be avoided as much as possible. Therefore, it is a waste of existing advantages to feed the gas which has already been separated back to the stock circulation. It is also a waste of stock if, on the other hand, all stock conveyed along with the gas were separated from the stock circulation by discharging it as a secondary flow of the pump.
Another disadvantage is that when the consistency of the pulp varies the amount of gas in the pulp also varies but at a much larger scale. Since the pump has usually, for practical reasons, been adjusted to remove nearly all the gas from the pulp, in a case when the amount of gas is at its minimum, all the gas exceeding that amount will be returned to the pulp flow. In some cases when the amount of gases is expected to vary at a large scale, more than half of the gas is returned back to the circulation.
The most disadvantageous feature of nearly all of the prior art gas discharge device has, however, been the separate vacuum pump having a separate driving motor with separate installation etc. A separate vacuum pump with a drive motor has added to the costs of constructions, which has been one of the obstacles to a wider acceptance of centrifugal pumps for stock handling. The present invention, however, has rendered possible the combination of a vacuum pump with the centrifugal pump impeller for removing gas from the pump.
U.S. Pat. No. 4,776,758 discloses a centrifugal pump having fluidizing vanes in front of the centrifugal impeller and a vacuum pump arranged in a separate chamber and on the same shaft with the impeller. Thus, a separate vacuum pump and drive motor have been omitted, but the structure of the pump itself is, however, complicated as both the vacuum impeller and the centrifugal impeller have housings of their own separated by a common wall member. Thus, the impellers are entirely separate structures and the common wall has to be manufactured as a separate part for practical reasons, as one has to be able to install the vacuum impeller on the shaft. The vacuum pump used in said patent is a so-called liquid ring pump.
One object of the present invention is to simplify even further the structure of a centrifugal pump having a gas separating vacuum pump arranged therein. A characterizing feature of the pump in accordance with the present invention is the combination of the centrifugal pump impeller with the vacuum pump impeller so that the vacuum impeller is arranged on the back side of the centrifugal impeller without the necessity of a separating wall. Another feature of the apparatus in accordance with the invention is the presence of several pressure areas or spaces each with differing pressure and located behind the impeller. The differing pressure areas are provided by arranging the clearances between the impeller back plate and the impeller back vanes with respect to their opposing or counter surfaces as small as possible thereby preventing the pressurized gas/liquid/gas containing medium from escaping therefrom. The spaces between the back vanes of the impeller are forming these differing pressure stages/areas by being sealed off as efficiently as possible by maintaining only small clearances between stationary and moving parts or by arranging the ends of the back vanes near the shaft of the pump by firmly and tightly attaching the vanes to an impeller hub portion extending substantially axially from the impeller back plate.
The advantages of the method and apparatus of the present invention are as follows:
a separate vacuum pump and its driving motor are not needed; PA1 the structural changes in the pump housing are minimal compared to the known MC-pumps; PA1 the manufacture of a separate vacuum pump impeller has been avoided; and PA1 a known MC-pump can be easily converted to include the new impeller and a vacuum pump housing in accordance with the present invention.